As is well known, the nozzle expansion of a rocket engine required for maximum thrust efficiency varies with the ambient pressure. In multi-stage rocket engines where a later stage is to be fired in space or at high altitudes, the expansion ratio is of major importance and requires a terminal divergent exit nozzle for the rocket engine of substantial length and exit diameter. However, to make a second or later engine stage of such overall length and diameter as would be required to produce a requisite expansion ratio would be extremely cumbersome and wasteful of otherwise usable space in the rocket assembly. Thus, it is desirable to provide means associated with a rocket engine so that a divergent nozzle of a length and diameter suitable for maximum thrust at low ambient pressure may be obtained by deploying such means substantially coincidental to firing of the engine. In this way, the inoperative rocket engine and its accessories may be compacted so that the entire assembly requires a storage space no larger than the non-extended rocket engine assembly.